European patent publication EP-A-0 005 281 discloses a liquid coating composition based on an ester thiol and an aminoplast curing agent. The aminoplast curing agent is based on, preferably etherified, N-methylol groups, which are obtained by the reaction of amino or amido groups-containing compounds and formaldehyde. Etherified N-methylol groups can be described as N,O-acetals. The ester thiol is built up on the one hand from one or more polyhydroxyl compounds, and on the other hand from one or more mercaptocarboxylic acids.
A drawback to coating compositions based on such curing agents comprising (etherified) N-methylol groups is the possible release of formaldehyde during application and curing, and even during use of the coated object. Due to the toxicity of formaldehyde, the release of even small amounts of this compound is highly undesirable.
The coating composition of EP-A-0 005 281 is cured at elevated temperature, between 60° and 160° C. Although curing within this temperature range may be acceptable for many purposes, the need to cure in an oven at elevated temperature undesirably limits the possible application areas of the coating composition. Thus the coating composition is not suitable for the (re)finishing of cars, large transportation vehicles, and airplanes.
Water borne coating compositions are not disclosed in EP-A-0 005 281.
Polymers and binders based on aminoacetals represented by formula I
wherein A is NH, n is an integer from 1 to 10 and R and R′ may be the same or different and represent alkyl groups with 1 to 4 carbon atoms, are known in the art, for example from United States patent publication U.S. Pat. No. 4,663,410 and European patent publication EP-A 1 050 550. Aminoacetals of formula I can be used for the preparation of acetal-functional binders, which are appreciated for their capacity to be cross-linked with one another and/or other polymer or substrate-reactive groups under mild conditions to give binders, adhesives and/or coatings with good water and solvent resistance and good substrate adhesion. Cross-linking reactions of acetal-functional binders are generally acid catalyzed. Therefore, in order to prevent an equimolar amount of acidic catalyst being necessary to neutralize the basic amino group of the aminoacetal, said amino group must be transformed into a non-basic group such as an amide group, urethane group, or urea group.
In U.S. Pat. No. 4,663,410 it is proposed to prepare polymerizable amide acetals by reacting aminoacetals with, e.g., (meth)acrylic acid chloride. Drawbacks to said known process are the use of the highly toxic (meth)acryloyl chloride and the formation of an equimolar amount of chloride salt. Moreover, the high polarity of the obtained acrylamidoacetal may give problems when it is used as a (co)monomer in polymerization processes.
Also the direct reaction of an aminoacetal with either a polymerizable monoisocyanate such as m-isopropenyl-dimethylbenzyl isocyanate or with a polyisocyanate, such as described in EP-A 1 050 550, is a problematic route towards acid curable acetal-functional binders, as said reaction is attended with the formation of urea groups which generally give rise to crystalline and/or sparingly soluble materials. Crystallinity and poor solubility render the preparation of acetal-functional binders difficult and further detract from the film forming properties of a coating composition containing such acetal-functional binders. These drawbacks become more apparent as a higher proportion of acetal-functional groups is introduced into the acetal-functional binder, as it will be the case in more demanding applications which call for a high cross-link density.